[llvm] [RemoveDIs][NFC] Introduce DbgRecord base class [1/3] (PR #78252)

[Jeremy Morse via llvm-commits]

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@@ -63,38 +65,92 @@ class DPMarker;
 class DPValue;
 class raw_ostream;
 
-/// Record of a variable value-assignment, aka a non instruction representation
-/// of the dbg.value intrinsic. Features various methods copied across from the
-/// Instruction class to aid ease-of-use. DPValue objects should always be
-/// linked into a DPMarker's StoredDPValues list. The marker connects a DPValue
-/// back to it's position in the BasicBlock.
+/// Base class for non-instruction debug metadata records that have positions
+/// within IR. Features various methods copied across from the Instruction
+/// class to aid ease-of-use. DbgRecords should always be linked into a
+/// DPMarker's StoredDPValues list. The marker connects a DbgRecord back to
+/// it's position in the BasicBlock.
 ///
-/// This class inherits from DebugValueUser to allow LLVM's metadata facilities
-/// to update our references to metadata beneath our feet.
-class DPValue : public ilist_node<DPValue>, private DebugValueUser {
-  friend class DebugValueUser;
-
-  // NB: there is no explicit "Value" field in this class, it's effectively the
-  // DebugValueUser superclass instead. The referred to Value can either be a
-  // ValueAsMetadata or a DIArgList.
+/// We need a discriminator for dyn/isa casts. In order to avoid paying for a
+/// vtable for "virtual" functions too, subclasses must add a new discriminator
+/// value (RecordKind) and cases to a few functions in the base class:
+///   deleteRecord()
+///   clone()
+///   both print methods
+class DbgRecord : public ilist_node<DbgRecord> {
+public:
+  /// Marker that this DbgRecord is linked into.
+  DPMarker *Marker = nullptr;
+  /// Subclass discriminator.
+  enum Kind : uint8_t { ValueKind };
 
-  DILocalVariable *Variable;
-  DIExpression *Expression;
+protected:
   DebugLoc DbgLoc;
+  Kind RecordKind; ///< Subclass discriminator.
 
 public:
-  void deleteInstr();
+  DbgRecord(Kind RecordKind, DebugLoc DL)
+      : DbgLoc(DL), RecordKind(RecordKind) {}
+
+  /// Methods requiring subclass implementations.
----------------
jmorse wrote:

First time I read this I guessed it meant "subclasses must define these methods", but what it really means is that there's a switch in the implementation on the kind? Some kind of rewording would be appreciated, not really sure how though.

https://github.com/llvm/llvm-project/pull/78252